With the quickly increased demands for all kinds of portable electronic products, such as digital camera, personal digital assistant (PDA), MP3, etc., there are also increasing demands for high-capacity rechargeable batteries that can be fully charged within a largely shortened time with a compact and lightweight charger that has low manufacturing cost and long service life.
While the currently available chargers have different advantages and disadvantages, most of them fail to meet the above-mentioned requirements of being compact, lightweight, economical, and durable. Therefore, it is still desirable to improve the conventional chargers.
FIG. 1 shows a charging circuit 12 for a conventional serial charger. As shown, an alternating current (AC) of 100V-240V is converted by a power converter 11 into a direct current (DC) charging voltage, which is used to charge a plurality of serially connected batteries B1, B2, B3, B4 via the charging circuit 12. While the above-described conventional serial charger has simple structure and low price, it does not ensure all the serially connected batteries are balance-charged. In other words, it is possible some of the serially connected batteries are fully charged while others are not. As a result, some of the batteries might be excessively charged to cause dangers, while others are still not fully charged when the charging stops; or, some of the batteries would become deteriorated and have reduced service life due to being repeatedly recharged or discharged.
FIG. 2 shows a charging circuit 12 for a conventional parallel charger. A plurality of parallelly connected batteries B1, B2, B3, B4 are charged via the charging circuit 12. With the parallel charger, the parallelly connected batteries have similar charging voltage without the risk of being excessively charged. However, the parallel charger fails to meet the requirement of quick charging. Moreover, it is uneasy to select and arrange suitable electronic control elements for the parallel charger. For instance, when the charging voltage is 3.7V, and each of the parallelly connected batteries has a charging current of 1.5 A, total 6 amperes of charging current is required. And, since voltage difference is existed among different transistors included in the charging circuit of the parallel charger, there are many difficulties to be overcome before a low-voltage high-current charging circuit can be formed. For example, since the wiring on the circuit board of the parallel charger must not be less than 6 mm to meet relevant safety code, the parallel charger usually has a large volume and the components thereof tend to produce high amount of heat. Moreover, it is uneasy to achieve the effect of high-volume quick charging with the parallel charger. Therefore, the conventional parallel charger no longer meets the market demands now. When it is desired to increase the charging voltage to thereby enhance the power and reduce the current of the parallel charger, the voltage differences among the switch transistors Q1, Q2, Q3, Q4 included in the charging circuit 12 would cause the tough problem of high temperature. In the event heat radiation elements are added in an attempt to radiate heat produced by the transistors, the parallel charger would have excessively large volume and largely increased manufacturing cost.
Therefore, the main goal of all charging circuits is trying to able to charge every single battery fully and equally in the circuit without some of batteries either over-charged or under-charged. The conventional dissipative balance charging and non-dissipative balance charging circuits are designed to improve the charging condition in the circuit.
A dissipative balance charging is disclosed in U.S. Patent Number 2005/0127873 A1, the battery A is set to a pre-determined valve, so once the voltage reaches to the pre-determined valve, the charge current in the circuit will change via the switch device. In other words, when the charge voltage of battery A reaches to the pre-determined valve, the charge current of the circuit will flow to the discharge resistor first to dissipate some of its voltage before passing through battery A so that the charge current flows to the battery A will be lowered than the charge current in battery B in order to provide a balance charging condition in the circuit. As a result, such charging circuit is designed to be used for the batteries that are connected in series. When only one of the batteries is fully charged in the circuit, the charge current will be discharged off partially or completely at the discharge resistor in order to prevent the fully charged battery from over-charging which can cause substantially damage to the battery.
However, the dissipative balance charging circuit disclosed in U.S. Patent Number 2005/0127873 A1 can cause the temperature in the circuit to rise when the charge current is discharged or dissipated at the discharge resistor because the discharge energy is transformed into the heat energy and the heat cannot be dissipated out easily which will result the charging circuit less reliable and become unstable. Further when a large amount of charge current is required, then the dissipative balance charging circuit will generate more heat energy to dissipate the charge current. Therefore, this kind of conventional circuit will not be suitable for the large charging current. Moreover, this kind of dissipative balance charging circuit cannot effectively utilize the energy.
An other type of conventional balance charging circuit is shown in FIG. 3. This non-dissipative balance charging circuit comprises a plurality of balance circuits, wherein the balance circuits are designed to charge the batteries connected in series respectively. Each battery set having several batteries is being charged independently by its corresponding balance circuit, wherein the balance circuit comprises two Field Effect Transistor (FET) Q11 & Q12, two diode D and once energy storage inductor L1. When a misbalance condition occurs due to the voltage different between two of the batteries of the battery balance circuit BC1, for example when the voltage of B1 is greater than voltage of B2, the FET Q11 will be switched on, and energy storage inductor L1 will be charged by the battery B1. When the FET Q11 is switched off, the energy storage inductor L1, the battery B2 and the diode D12 will be connected. As a result, the energy stored at the energy storage inductor L1 will be transferred to the battery B2. Similarly, during the charging operation, when the voltage of B2 is greater than the voltage of B1, the switching states of the FET Q12 will be used to transfer energy from battery B2 to battery B1. Therefore, this type of balance charging circuit is a balance device of energy bi-directional transmission by transferring energy from high voltage to the low voltage in order to achieve the equilibrium in the circuit.
Therefore, the conventional non-dissipative balance charging circuit is complex and comprises more devices than most other types of charging circuits. Since each device installed in the circuit will affect the stability of the whole circuit, therefore, the more complex and cumbersome of the devices are, the less stable and reliable of the circuit will be. Furthermore, the cost of this kind of charging circuit is high and the size of the circuit is not compactly built.